1. Field of the Invention
The present invention relates in general to the field of electronics, and more specifically to a system and method of reducing supply current variations using compensation current control.
2. Description of the Related Art
Many electronic systems utilize switching power converters to efficiently convert power from one source into power useable by a device (referred to herein as a “load”). For example, power companies often provide alternating current (AC) power at specific voltages within a specific frequency range. However, many loads utilize power at a different voltage and/or frequency than the supplied power. For example, some loads, such as light emitting diode (LED) based lamps operate from a direct current (DC). “DC current” is also referred to as “constant current”. “Constant” current does not mean that the current cannot change over time. The DC value of the constant current can change to another DC value. Additionally, a constant current may have noise or other minor fluctuations that cause the DC value of the current to fluctuate. “Constant current devices” have a steady state output that depends upon the DC value of the current supplied to the devices.
LEDs are becoming particularly attractive as main stream light sources in part because of energy savings through high efficiency light output, long life, and environmental incentives such as the reduction of mercury. LEDs are semiconductor devices and are best driven by direct current. The brightness of the LED varies in direct proportion to the DC current supplied to the LED. Thus, increasing current supplied to an LED increases the brightness of the LED and decreasing current supplied to the LED dims the LED.
FIG. 1 depicts a power distribution system 100 that includes a leading edge, phase-cut dimmer 102. FIG. 2A depicts ideal, exemplary voltage graphs 200 associated with the power distribution system 100. Referring to FIGS. 1 and 2, the lighting system 100 receives an AC supply voltage VIN from voltage supply 104. The supply voltage VIN, indicated by voltage waveform 202, is, for example, a nominally 60 Hz/110 V line voltage in the United States of America or a nominally 50 Hz/220 V line voltage in Europe. A leading edge dimmer 102 phase cuts leading edges, such as leading edges 204 and 206, of each half cycle of supply voltage VIN. Since each half cycle of supply voltage VIN is 180 degrees of the input supply voltage VIN, the leading edge dimmer 102 phase cuts the supply voltage VIN at an angle greater than 0 degrees and less than 180 degrees. Generally, the voltage phase cutting range of a leading edge dimmer 102 is 10 degrees to 170 degrees. “Phase cutting” the supply voltage refers to modulating a leading edge phase angle of each cycle of an alternating current (“AC”) supply voltage. “Phase cutting” of the supply voltage is also commonly referred to as “chopping”. Phase cutting the supply voltage reduces the average power supplied to a load, such as a lighting system, and thereby controls the energy provided to the load.
The input signal voltage Vφ—IN to the lighting system 100 represents a dimming level that causes the lighting system 100 to adjust power delivered to a lamp 122, and, thus, depending on the dimming level, increase or decrease the brightness of the lamp 122. Many different types of dimmers exist. In general, dimmers use a digital or analog coded dimming signal that indicates a desired dimming level. For example, the triac-based dimmer 102 phase cuts a leading edge of the AC input supply voltage VIN. The leading edge dimmer 102 can be any type of leading edge dimmer such as a triac-based leading edge dimmer available from Lutron Electronics, Inc. of Coopersberg, Pa. (“Lutron”). A triac-based leading edge dimmer is described in the Background section of U.S. patent application Ser. No. 12/858,164, entitled Dimmer Output Emulation, filed on Aug. 17, 2010, and inventor John L. Melanson.
The phase cut dimmer 102 supplies the input voltage Vφ—IN as modified by the phase cut dimmer 102 to a full bridge diode rectifier 107. The full bridge rectifier 107 supplies an AC rectified voltage, which is represented by the voltage supply 109 to the switching power converter 108. Capacitor 105 filters high frequency components from rectified voltage VφR—IN. To control the operation of switching power converter 108, controller 106 generates a control signal CS1 to control conductivity of field effect transistor (FET) switch 112 to control conductivity of the current control switch 112 of flyback-type switching power converter 110 to control the conversion of the input voltage VIN into a secondary voltage VS. When control signal CS1 causes switch 112 to conduct, a primary-side current iPRIMARY flows into a primary coil 114 of transformer 116 to magnetize the primary coil 114. When control signal CS1 opens switch 112, primary coil 114 demagnetizes. The magnetization and demagnetization of the primary coil 114 induces a secondary voltage VS across a secondary coil 118 of transformer 116. Primary voltage VP is N times the secondary voltage VS, i.e. VP=N·VS, and “N” is a ratio of coil turns in the primary coil 114 to the coil turns in the secondary coil 118. The secondary-side current iSECONDARY is a direct function of the secondary voltage VS and the impedance of diode 120, capacitor 122, and load 104. Diode 120 allows the secondary-side current iSECONDARY to flow in one direction. The secondary-side current iSECONDARY charges capacitor 122, and capacitor 122 maintains an approximately DC voltage VLOAD across load 104. Thus, secondary-side current iSECONDARY is a DC current. Waveforms 104 depict exemplars of control signal CS1, primary-side current iprimary, and secondary-side current isecondary. It is commonly assumed that the secondary-side current iSECONDARY rises virtually instantaneously after the primary-side winding 114 stops conducting the primary-side current iPRIMARY.
Since the control signal CS1 generated by the controller 106 controls the primary-side current iPRIMARY, and the primary-side current iPRIMARY controls the voltage VP across the primary coil 114, the energy transfer from the primary coil 114 to the secondary coil 118 is controlled by the controller 106. Thus, the controller 106 controls the secondary-side current iSECONDARY.
The controller 106 operates the switching power converter 110 in a certain mode, such as quasi-resonant mode. In quasi-resonant mode, the control signal CS1 turns switch 112 ON at a point in time that attempts to minimize the voltage across switch 112, and, thus, minimize current through switch 112. Controller 106 generates the control signal CS1 in accordance with a sensed primary-side current iPRIMARY—SENSE, obtained via signal iLINK—SENSE from link current sense path 126.
Switching power converters convert power received from a power source, such as a voltage supply, into power suitable for a load. The power received from the voltage supply is referred to as “POWER IN”, and the power provided to the load is referred to as “POWER OUT”. All switching power converters have some inherent power losses due to, for example, non-ideal component characteristics. Such inherent power losses tend to be minimized so as to increase the efficiency of the switching power converters. Inherent power losses are represented herein by “PINH”. In some contexts, the amount of power supplied to the switching power converter can exceed the amount of power provided by the switching power converter to a load, i.e. POWER IN>POWER OUT+PINH. When the POWER IN is greater than the POWER OUT+PINH, the switching power converter passively dissipates the excess energy using passive resistors.
A dimmable lighting system that includes a low power lamp as the load 104, such as one or more light emitting diodes (LEDs), represents one context when the POWER IN to the switching power converter can be greater than the POWER OUT+PINH of the switching power converter. In this exemplary context, the switching power converter receives current through a triode for alternating current (“triac”) based dimmer 102. Once the triac-based dimmer 102 begins conducting during a cycle of an alternating current (“AC”) supply voltage VIN to prevent the triac from disadvantageously, prematurely disconnecting during mid-cycle of the supply voltage, the switching power converter 110 draws a minimum current referred to as a “hold current.” As long as an input current iPRIMARY to the switching power converter 110 is greater than or equal to the hold current, the triac-based dimmer 102 should not prematurely disconnect. For a leading edge dimmer, a premature disconnect occurs when the dimmer begins conducting and stops conducting prior to reaching a zero crossing of the supply voltage. Premature disconnects can cause problems with the lighting system, such as flicker and instability.
Thus, to prevent premature disconnection of the triac-based dimmer, the minimum POWER IN to the switching power converter 110 equals the hold current (“iHOLD”) times an input voltage “VIN” to the switching power converter 110.